1. Field of the Invention
This invention relates to slotted array antennas and more particularly to a dual end resonant slot array feed for a resonant slotted waveguide planar array antenna.
2. Brief Description of the Prior Art
In the past slotted array antennae have been fed by single end feed mechanisms. When a waveguide section is fed at one end a waveguide short at the opposite end, sets up a standing wave in the waveguide. Shunt or series slot elements are located at appropriate points on the standing wave pattern (voltage or current peaks, respectively) to cause radiation with the correct amplitude and phase. Over a band of frequencies, the standing wave pattern in the waveguide varies relative to the location of the slots, causing errors in the slot amplitudes and phases. The magnitude of these errors increases in a direct relationship to the deviation of frequency from the design center frequency. The magnitude of the errors also increases with the length of the waveguide, and hence the number of slots. For waveguides having four or more slots, the usable bandwidth of a single end feed is on the order of .+-.1 percent.
To improve the bandwidth relative to a single end feed, E-plane and H-plane tee feeds have been used. The E-plane tee feed is in essence, two single end feeds joined at their respective feed points by an E-plane waveguide tee; improvement is caused by reducing the length (and number of slots) associated with each of the two single end feeds. The problem with the E-plane feed is that in order to maintain equal slot spacing one slot must lie directly under the E-plane tee. Owing to mutual coupling to the E-plane tee, this slot suffers a variation in phase and amplitude over the frequency band which differs significantly from the other slots in the array. This significantly different set of phase/amplitude errors for the slot under the E-plane feed largely offsets any bandwidth advantages that otherwise would have been obtained by using the E-plane tee.
By substituting an H-plane (shunt) tee for the E-plane (series) tee, the feed point for the slot waveguide can be located half way between two slots instead of directly over the slots. Nevertheless, as the H-plane feed must be about one-half wavelength wide (to avoid waveguide cutoff effects), the feed then couples to the two slots adjacent to the feed, yielding essentially the same bandwidth limitations as the E-plane feed.
For a large array antenna, the bandwidth typically has been limited to less than 2.5% using one of the above methods owing to the need to keep the manifold complexity within reasonable bounds. Both the amplitude and phase of the aperture illumination begin to be significantly degraded at +1% of the center frequency. The single end feed for a resonant waveguide array is described in a number of texts on antennas. Those persons skilled in the art desiring more detailed information pertaining to single end feeds are referred to Johnson and Jasik's "Antenna Engineering Handbook", Second Edition, 1984 and 1961, Chapter 9.